Stacked battery

ABSTRACT

A main object of the present disclosure is to provide a stacked battery in which an unevenness of short circuit resistance among a plurality of cells is suppressed. The present disclosure achieves the object by providing a stacked battery comprising: a plurality of cells in a thickness direction, wherein the plurality of cells are electrically connected in parallel; each of the plurality of cells includes a cathode current collector, a cathode active material layer, a solid electrolyte layer, an anode active material layer, and an anode current collector, in this order; the stacked battery includes a surface-side cell that is located on a surface side of the stacked battery, and a center-side cell that is located on a center side rather than the surface-side cell; and a contact resistance between the cathode current collector and the anode current collector in the surface-side cell is more than a contact resistance between the cathode current collector and the anode current collector in the center-side cell.

TECHNICAL FIELD

The present disclosure relates to a stacked battery.

BACKGROUND ART

Stacked batteries comprising a plurality of cells in a thicknessdirection are known; wherein each of the plurality of cells includes acathode current collector, a cathode active material layer, a solidelectrolyte layer, an anode active material layer, and an anode currentcollector, in this order. For example, Patent Literature 1 discloses alithium ion secondary battery comprising a plurality of unit cells,wherein each of the plurality of unit cells includes: a cathode layerprovided with a cathode current collector and a cathode mixture layer; asolid electrolyte layer; and an anode layer provided with an anodecurrent collector and an anode mixture layer. Further, Patent Literature1 discloses a nail penetration test as a method for evaluating thesafety of all solid batteries.

Also, for example, Patent Literature 2 discloses a method for producinga stacked type all solid battery wherein: the stacked type all solidbattery comprises a plurality of all solid battery cells connected in abipolar form or in a monopolar form; and each of the plurality of allsolid battery cells includes a cathode current collector layer, acathode active material layer, a solid electrolyte layer, an anodeactive material layer, and an anode current collector layer.

CITATION LIST Patent Literatures

Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No.2016-207614

Patent Literature 2: JP-A No. 2016-136490

SUMMARY OF DISCLOSURE Technical Problem

As described above, the nail penetration test is known as a method forevaluating the safety of all solid batteries. The nail penetration testis a test of penetrating a conductive nail through an all solid battery,and observing changes (such as a temperature change) when an internalshort circuit within the battery occurs.

From detailed studies of the nail penetration test of stacked batteriescomprising a plurality of all solid battery cells electrically connectedin parallel, the present inventors have acquired new knowledge that theresistance of a short circuit part (short circuit resistance) in eachcell varies greatly with the cell location. When a cell with low shortcircuit resistance and a cell with high short circuit resistance aremixed, a current flows from the cell with high short circuit resistanceinto the cell with low short circuit resistance. Hereinafter, this maybe referred to as a “sneak current”. When the sneak current occurs, thetemperature of the cell with low short circuit resistance (the cell towhich the current flowed into) increases, and as the result, the batterymaterials are easily deteriorated.

The present disclosure has been made in view of the above circumstances,and a main object thereof is to provide a stacked battery in which theunevenness of short circuit resistance among a plurality of cells issuppressed.

Solution to Problem

In order to achieve the object, the present disclosure provides astacked battery comprising: a plurality of cells in a thicknessdirection, wherein the plurality of cells are electrically connected inparallel; each of the plurality of cells includes a cathode currentcollector, a cathode active material layer, a solid electrolyte layer,an anode active material layer, and an anode current collector, in thisorder; the stacked battery includes a surface-side cell that is locatedon a surface side of the stacked battery, and a center-side cell that islocated on a center side rather than the surface-side cell; and acontact resistance between the cathode current collector and the anodecurrent collector in the surface-side cell is more than a contactresistance between the cathode current collector and the anode currentcollector in the center-side cell.

In the disclosure, at least one of the cathode current collector and theanode current collector in the surface-side cell may include, on asurface thereof, an oxide layer.

In the disclosure, the cathode current collector in the surface-sidecell may include, on a surface thereof, a first coating layer containinga carbon material, and the cathode current collector in the center-sidecell may include, on a surface thereof, a second coating layercontaining a carbon material.

In the disclosure, a content of the carbon material in the first coatinglayer may be less than a content of the carbon material in the secondcoating layer.

In the disclosure, a thickness of the first coating layer may be morethan a thickness of the second coating layer.

In the disclosure, when a proportion of a resistance R₂₀₀ at 200° C.with respect to a resistance R₂₅ at 25° C. is R₂₀₀/R₂₅, the R₂₀₀/R₂₅value of the first coating layer may be more than the R₂₀₀/R₂₅ value ofthe second coating layer.

In the disclosure, the anode current collector in the surface-side cellmay include, on a surface thereof, a third coating layer containing acarbon material, and the anode current collector in the center-side cellmay include, on a surface thereof, a fourth coating layer containing acarbon material.

In the disclosure, a content of the carbon material in the third coatinglayer may be less than a content of the carbon material in the fourthcoating layer.

In the disclosure, a thickness of the third coating layer may be morethan a thickness of the fourth coating layer.

In the disclosure, when a proportion of a resistance R₂₀₀ at 200° C.with respect to a resistance R₂₅ at 25° C. is R₂₀₀/R₂₅, the R₂₀₀/R₂₅value of the third coating layer may be more than the R₂₀₀/R₂₅ value ofthe fourth coating layer.

In the disclosure, one of the cathode current collector and the anodecurrent collector in the surface-side cell may include, on a surfacethereof, a coating layer containing a carbon material, and another maynot include, on a surface thereof, a coating layer containing a carbonmaterial; and one of the cathode current collector and the anode currentcollector in the center-side cell may include, on a surface thereof, acoating layer containing a carbon material, and another may not include,on a surface thereof, a coating layer containing a carbon material inreverse to the surface-side cell.

According to the present disclosure, since a contact resistance betweenthe cathode current collector and the anode current collector in thesurface-side cell is more than a contact resistance between the cathodecurrent collector and the anode current collector in the center-sidecell, the unevenness of short circuit resistance among the plurality ofcells may be suppressed in the stacked battery.

In the disclosure, when each of the plurality of cells is numbered as1^(st) cell to N^(th) cell, in which N≥3, in order along the thicknessdirection of the stacked battery, the surface-side cell may be a cellthat belongs to a cell region A including 1^(st) cell to (N/3)^(th)cell.

In the disclosure, the center-side cell may be a cell that belongs to acell region B including ((N/3)+1)^(th) cell to (2N/3)^(th) cell.

In the disclosure, an average value of the contact resistance in thecell region A may be more than an average value of the contactresistance in the cell region B.

In the disclosure, when each of the plurality of cells is numbered as1^(st) cell to N^(th) cell, in which N≥60, in order along the thicknessdirection of the stacked battery, the surface-side cell may be a cellthat belongs to a cell region C including 1^(st) cell to 20^(th) cell.

In the disclosure, the center-side cell may be a cell that belongs to acell region D including 21^(st) cell to 40^(th) cell.

In the disclosure, an average value of the contact resistance in thecell region C may be more than an average value of the contactresistance in the cell region D.

In the disclosure, the anode active material layer may include Si or aSi alloy as an anode active material.

Advantageous Effects of Disclosure

The stacked battery in the present disclosure effects that an unevennessof short circuit resistance among a plurality of cells is suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of thestacked battery of the present disclosure.

FIG. 2 is a schematic cross-sectional view explaining a nail penetrationtest.

FIG. 3 is a graph showing the relationship between the cell location andthe short circuit resistance.

FIG. 4 is an equivalent circuit explaining a sneak current.

FIG. 5 is a schematic cross-sectional view explaining a nail penetrationtest.

FIGS. 6A to 6E are schematic cross-sectional views exemplifying a methodfor producing a two-stacked cell.

FIG. 7 is a graph exemplifying a voltage profile in a nail penetrationtest.

FIG. 8 is a schematic cross-sectional view explaining a testing methodof a contact resistance test.

FIG. 9 is a schematic cross-sectional view explaining a testing methodof a current breaking performance test.

DESCRIPTION OF EMBODIMENTS

The stacked battery of the present disclosure will be hereinafterdescribed in detail. FIG. 1 is a schematic cross-sectional view showingan example of the stacked battery of the present disclosure. Stackedbattery 100 shown in FIG. 1 comprises plurality of cells 10 (10A, 10B to10H to 10N) in a thickness direction; and each of plurality of cells 10includes cathode current collector 4, cathode active material layer 1,solid electrolyte layer 3, anode active material layer 2, and anodecurrent collector 5, in this order. Further, plurality of cells 10 areelectrically connected in parallel. A method for connecting the cells inparallel is not particularly limited, and for example, cell 10A and cell10B shown in FIG. 1 are connected in parallel in a manner that the cellsshare anode current collector 5. Incidentally, two cells next to eachother may or may not share cathode current collector 4 or anode currentcollector 5. In the latter case, for example, by providing two-layeredcathode current collector 4 or two-layered anode current collector 5,the two cells next to each other have cathode current collector 4 oranode current collector 5 individually between the cells.

Also, stacked battery 100 includes surface-side cell 10X that is locatedon a surface side of stacked battery 100, and center-side cell 10Y thatis located on a center side rather than surface-side cell 10X. Further,stacked battery 100 features a configuration that a contact resistancebetween cathode current collector 4 and anode current collector 5 insurface-side cell 10X is more than a contact resistance between cathodecurrent collector 4 and anode current collector 5 in center-side cell10Y.

According to the present disclosure, since a contact resistance betweenthe cathode current collector and the anode current collector in thesurface-side cell is more than a contact resistance between the cathodecurrent collector and the anode current collector in the center-sidecell, the unevenness of short circuit resistance among the plurality ofcells may be suppressed in the stacked battery. As described above, fromdetailed studies of the nail penetration test of stacked batteriescomprising a plurality of cells electrically connected in parallel, thepresent inventors have acquired new knowledge that the resistance of ashort circuit part (short circuit resistance) in each cell variesgreatly with the cell location.

This new knowledge will be explained referring to FIG. 2. As shown inFIG. 2, nail 110 is penetrated into stacked battery 100 comprisingplurality of cells 10 (10A, 10B to 10H to 10N) electrically connected inparallel. On this occasion, short circuit resistance R (R_(A), R_(B) toR_(H) to R_(N)) is determined with respect to each cell 10. As theresult of such detailed studies, as shown in FIG. 3 for example, it wasfound out that cell 10A located on the surface side has lower shortcircuit resistance compared to cell 10H located on the center side. Inother words, it was found out that there was the unevenness of shortcircuit resistance among the plurality of cells.

When a cell with low short circuit resistance and a cell with high shortcircuit resistance are mixed, a current flows from the cell with highshort circuit resistance into the cell with low short circuitresistance. As shown in FIG. 4 for example, when a short circuit occurswithin a stacked battery comprising cell 10A and cell 10H electricallyconnected in parallel in which short circuit resistance R_(A) of cell10A is lower than short circuit resistance R_(H) of cell 10H, sneakcurrent I flowing from cell 10H into cell 10A occurs in accordance withOhm's law. When sneak current I occurs, the temperature of cell 10Arises due to Joule heating; as the result, the deterioration of thebattery material easily occurs.

Although the reason why the unevenness of short circuit resistanceexists among the plurality of cells is not completely clear, it ispresumed as follows. As shown in FIG. 5 for example, on the surface side(such as location P₁ in FIG. 3) of the stacked battery, by penetratingnail 110 into cell 10, a state in which cathode current collector 4 andanode current collector 5 are in contact, and a state in which cathodeactive material layer 1 and anode current collector 5 are in contact,are presumed to occur.

Meanwhile, on the center side (such as location P₂ in FIG. 3) of thestacked battery, since the nail proceeds while dragging the fragment ofeach member, a state in which the cathode current collector and theanode current collector are not in contact, and a state in which thecathode active material layer and the anode current collector are not incontact, are presumed to occur. For the “state not in contact”, forexample, the following states may be supposed: a state in which thefragment of the solid electrolyte layer exists between the two, and astate in which a void exists between the two. As the result, the shortcircuit resistance will be higher on the center side of the stackedbattery.

Incidentally, the behavior of the short circuit resistance on thesurface side that is opposite to the nail penetrating surface (such aslocation P₃ in FIG. 3) of the stacked battery may possibly vary with theconstitution of the stacked battery; however, in both of the laterdescribed Reference Examples 1 and 2, the short circuit resistance waslowered. The reason therefor is presumed that, since the nail proceedswhile dragging the larger amount of the fragment of each member, thecathode current collector and the anode current collector will be in astate electrically connected by the fragment with high electronconductivity.

In contrast, in the present disclosure, since a contact resistancebetween the cathode current collector and the anode current collector inthe surface-side cell is more than a contact resistance between thecathode current collector and the anode current collector in thecenter-side cell, the unevenness of short circuit resistance among theplurality of cells may be suppressed in the stacked battery.Specifically, by using a cell with relatively high contact resistancefor the surface-side cell with low short circuit resistance, and using acell with relatively low contact resistance for the center-side cellwith high short circuit resistance, the unevenness of short circuitresistance among the plurality of cells may be suppressed. Incidentally,in the present disclosure, an inclusive term of merely “currentcollector” may be used for a cathode current collector and an anodecurrent collector.

Also, the problem of suppressing the unevenness of short circuitresistance among the plurality of cells is a problem never occurs in asingle cell, that is, a problem peculiar to a stacked battery. Further,in a typical all-solid-type stack battery, since all of the constitutingmembers are solids, a pressure applied to the stacked battery during anail penetration test will be extremely high. Since a high pressure suchas 100 MPa or more at the part where the nail penetrates, andparticularly, 400 MPa or more at the tip part of the nail, is applied,the management of the short circuit resistance in a high pressurecondition is important. In contrast, in a liquid-based battery, since avoid into where the liquid electrolyte penetrates exists in theelectrode, a pressure applied to the battery during a nail penetrationtest will be greatly lower. That is, it is difficult to conceive ofmanaging the short circuit resistance in a high pressure condition,based on the technique of the liquid-based battery.

1. Contact Resistance Between Cathode Current Collector and AnodeCurrent Collector

The stacked battery of the present disclosure includes a surface-sidecell that is located on a surface side of the stacked battery, and acenter-side cell that is located on a center side rather than thesurface-side cell. Further, a contact resistance between the cathodecurrent collector and the anode current collector in the surface-sidecell is more than a contact resistance between the cathode currentcollector and the anode current collector in the center-side cell.Particularly, the difference of the contact resistances in a highpressure condition (such as 100 MPa) is preferably large.

The stacked battery of the present disclosure usually comprises twokinds or more of the cells with a different contact resistance betweenthe cathode current collector and the anode current collector. Here,“surface-side cell” and “center-side cell” in the present disclosure arestipulations for specifying the cells with a different contactresistance between the cathode current collector and the anode currentcollector. For example, an assumed case is a stacked battery comprisingtwo kinds of the cells (cell α and cell β) with a different contactresistance between the cathode current collector and the anode currentcollector. Incidentally, the contact resistances are cell α>cell β. Whena plurality of cells α are stacked on the surface side of the stackedbattery, any one of the plurality of cells α may be specified as thesurface-side cell. Meanwhile, when a plurality of cells β are stacked onthe center side of the stacked battery, any one of the plurality ofcells β may be specified as the center-side cell. Also, when the stackedbattery comprises three kinds or more of the cells with a differentcontact resistance between the cathode current collector and the anodecurrent collector, comparing two cells with a different contactresistance among them, when the magnitude relation of the contactresistances and the locational relation of the two cells satisfy thespecific conditions, one cell is specified as the surface-side cell, andthe other cell is specified as the center-side cell.

The contact resistance between the cathode current collector and theanode current collector in the surface-side cell is regarded as R_(c1),and the contact resistance between the cathode current collector and theanode current collector in the center-side cell is regarded as R_(c2).The value of R_(c1)/R_(c2) under pressure of 100 MPa is, for example,1.5 or more, and may be 10 or more. Meanwhile, the value ofR_(c1)/R_(c2) under pressure of 100 MPa is, for example, 10000 or less.Also, the value of R_(c1) under pressure of 100 MPa is not particularlylimited, and is, for example, 0.037 Ω·cm² or more, may be 0.047 Ω·cm² ormore, and may be 0.103 Ω·cm² or more. Meanwhile, the value of R_(c1)under pressure of 100 MPa is, for example, 0.277 Ω·cm² or less.Incidentally, a method for measuring a contact resistance between thecathode current collector and the anode current collector will beexplained in the later described Reference Example 1.

In the present disclosure, the contact resistance between the cathodecurrent collector and the anode current collector in the surface-sidecell is more than the contact resistance between the cathode currentcollector and the anode current collector in the center-side cell. Thecontact resistance may be adjusted, for example, by at least one of theproperties of the current collector, the properties of an oxide layerformed on a surface of the current collector, and the properties of acoating layer, containing a carbon material, formed on a surface of thecurrent collector.

(1) Property of Current Collector

The material for the cathode current collector in the surface-side cellmay be different from the material for the cathode current collector inthe center-side cell. Similarly, the material for the anode currentcollector in the surface-side cell may be different from the materialfor the anode current collector in the center-side cell. When materialsfor the current collectors are different, the contact resistance betweenthe cathode current collector and the anode current collector will bedifferent. “Different material” refers to a case in which at least oneof the followings in relation to the current collector is different:included elements, composition, and crystal state. In the surface-sidecell and the center-side cell, the cathode current collectors maycontain the same material whereas the anode current collectors maycontain different materials; the cathode current collectors may containdifferent materials whereas the anode current collectors may contain thesame material; and both of the cathode current collectors and the anodecurrent collectors may contain different materials.

(2) Property of Oxide Layer

At least one of the cathode current collector and the anode currentcollector in the surface-side cell may include, on a surface thereof, anoxide layer. A surface resistance increases by including the oxidelayer, and the contact resistance between the cathode current collectorand the anode current collector also increases. The oxide layer ispreferably formed on at least the solid electrolyte layer facing side ofthe current collector. Also, the oxide layer is preferably, for example,an oxide layer including the same metal element as the currentcollector. The thickness of the oxide layer is, for example, 10 nm ormore, and may be 20 nm or more. Also, the oxide layer is preferably anoxidation treated layer of the current collector. The oxidationtreatment may include, for example, a heat treatment in the atmosphere.The heat treatment temperature is, for example, 150° C. or more, and maybe within a range of 200° C. to 500° C.

At least one of the cathode current collector and the anode currentcollector in the center-side cell may include, on a surface thereof, anoxide layer. The thickness of the oxide layer in the cathode currentcollector of the surface-side cell is preferably lager than thethickness of the oxide layer in the cathode current collector of thecenter-side cell. Similarly, the thickness of the oxide layer in theanode current collector of the surface-side cell is preferably largerthan the thickness of the oxide layer in the anode current collector ofthe center-side cell. Meanwhile, the cathode current collector and theanode current collector in the center-side cell may not include theoxide layer on a surface thereof.

(3) Property of Coating Layer Containing Carbon Material

At least one of the cathode current collector and the anode currentcollector in the surface-side cell may include, on a surface thereof, acoating layer containing a carbon material. Also, at least one of thecathode current collector and the anode current collector in thecenter-side cell may include, on a surface thereof, a coating layercontaining a carbon material. Here, the coating layer of the cathodecurrent collector of the surface-side cell may be regarded as a firstcoating layer, the coating layer of the cathode current collector of thecenter-side cell may be regarded as a second coating layer, the coatinglayer of the anode current collector of the surface-side cell may beregarded as a third coating layer, and the coating layer of the anodecurrent collector of the center-side cell may be regarded as a fourthcoating layer, in some cases.

The coating layer is a layer containing at least a carbon material, andmay further contain a resin. Examples of the carbon material may includecarbon blacks such as furnace black, acetylene black, Ketjen black, andthermal black; carbon fibers such as carbon nanotube and carbonnanofiber; activated carbon; carbon; graphite; graphene; and fullerene.Examples of the shape of the carbon material may include a granularshape.

Examples of the resin may include a thermoplastic resin. Examples of thethermoplastic resin may include polyvinylidene fluorid (PVDF),polypropylene, polyethylene, polyvinyl chloride, polystyrene,acrylonitrile butadiene styrene (ABS) resin, methacrylic resin,polyamide, polyester, polycarbonate, and polyacetal. The meltingtemperature of the resin is, for example, within a range of 80° C. to300° C.

The thickness of the coating layer is, for example, within a range of 1μm to 20 μm, and may be within a range of 1 μm to 10 μm.

A content of the carbon material in the first coating layer may be lessthan a content of the carbon material in the second coating layer.Similarly, a content of the carbon material in the third coating layermay be less than a content of the carbon material in the fourth coatinglayer. A surface resistance increases by decreasing the content of thecarbon material in the coating layer of the surface-side cell, and thecontact resistance between the cathode current collector and the anodecurrent collector in the surface-side cell also increases. The contentof the carbon material in the first coating layer or the third coatinglayer is, for example, 20% by volume or less, and may be 10% by volumeor less. The cathode current collectors of the surface-side cell and thecenter-side cell may include the same material, whereas the contents ofthe carbon material in the coating layers may be different. Similarly,the anode current collectors of the surface-side cell and thecenter-side cell may include the same material, whereas the contents ofthe carbon material in the coating layers may be different.

A thickness of the first coating layer may be more than a thickness ofthe second coating layer. Similarly, a thickness of the third coatinglayer may be more than a thickness of the fourth coating layer. Thecontact resistance between the cathode current collector and the anodecurrent collector in the surface-side cell increases by increasing thethickness of the coating layer of the surface-side cell. The differenceof the thicknesses of the first coating layer and the second coatinglayer is, for example, 1 μm or more, may be 3 μm or more, and may be 6μm or more. The difference of the thicknesses of the third coating layerand the fourth coating layer is similar to the above. The cathodecurrent collectors of the surface-side cell and the center-side cell mayinclude the same material, whereas the thickness of the coating layersmay be different. Similarly, the anode current collectors of thesurface-side cell and the center-side cell may include the samematerial, whereas the thicknesses of the coating layers may bedifferent.

The coating layer may or may not further contain an inorganic filler. Inthe latter case, a coating layer with high electron conductivity may beobtained, and in the former case, a coating layer with PTC property maybe obtained. PTC means Positive Temperature Coefficient, and is aproperty whose resistance changes with a positive coefficient accordingto a temperature increase. Here, the resin contained in the coatinglayer may possibly expand in its volume according to a temperatureincrease and cause a resistance increase of the coating layer. However,in stacked batteries using all solid battery cells, since a bindingpressure is usually applied along the thickness direction, the resininfluenced by the binding pressure deforms or flows so that sufficientPTC property cannot be exhibited in some cases. Meanwhile, by adding ahard inorganic filler to the coating layer, preferable PTC property maybe exhibited even when influenced by the binding pressure. That is, thecurrent breaking performance is high. The binding pressure is, forexample, 0.1 MPa or more, may be 1 MPa or more, and may be 5 MPa ormore. Meanwhile, the binding pressure is, for example, 100 MPa or less,may be 50 MPa or less, and may be 20 MPa or less.

Examples of the inorganic filler may include a metallic oxide and ametallic nitride. Examples of the metallic oxide may include alumina,zirconia, and silica, and examples of the metallic nitride may includesilicon nitride. The average particle size (D₅₀) of the inorganic filleris, for example, within a range of 50 nm to 5 μm, and may be within arange of 100 nm to 2 μm. Also, the content of the inorganic filler inthe coating layer is, for example, 50% by volume or more, and may be 60%by volume or more. Meanwhile, the content of the inorganic filler in thecoating layer is, for example, 85% by volume or less, and may be 80% byvolume or less.

The proportion of resistance R₂₀₀ of the coating layer at 200° C. withrespect to resistance R₂₅ of the coating layer at 25° C. is regarded asR₂₀₀/R₂₅. The R₂₀₀/R₂₅ value of the first coating layer may be more thanthe R₂₀₀/R₂₅ value of the second coating layer. Similarly, the R₂₀₀/R₂₅value of the third coating layer may be more than the R₂₀₀/R₂₅ value ofthe fourth coating layer. The contact resistance between the cathodecurrent collector and the anode current collector in the surface-sidecell increases when the current breaking performance of the coatinglayer of the surface-side cell is high. The R₂₀₀/R₂₅ value ispreferably, for example, 10 or more. The cathode current collectors ofthe surface-side cell and the center-side cell may include the samematerial, whereas the current breaking performances of the coatinglayers may be different. Similarly, the anode current collectors of thesurface-side cell and the center-side cell may include the samematerial, whereas the current breaking performances of the coatinglayers may be different.

Also, one of the cathode current collector and the anode currentcollector in the surface-side cell may include, on a surface thereof, acoating layer containing a carbon material, and another may not include,on a surface thereof, a coating layer containing a carbon material;whereas one of the cathode current collector and the anode currentcollector in the center-side cell may include, on a surface thereof, acoating layer containing a carbon material, and another may not include,on a surface thereof, a coating layer containing a carbon material inreverse to the surface-side cell. In this case, both of the cathodecurrent collectors and the anode current collectors of the surface-sidecell and the center-side cell may include the same material, whereas thecurrent collectors on which the coating layers are formed may bedifferent.

2. Constitution of Stacked Battery

Each of the plurality of cells included in the stacked battery isnumbered as a 1^(st) cell to a N^(th) cell in order along the thicknessdirection of the stacked battery. N refers to the total cell numberincluded in the stacked battery; for example, N is 3 or more, may be 10or more, may be 30 or more, and may be 50 or more. Meanwhile, N is, forexample, 200 or less, may be 150 or less, and may be 100 or less.

The surface-side cell is preferably a cell that belongs to a cell regionincluding the 1^(st) cell to a (N/3)^(th) cell. Here, the (N/3)^(th)cell is a cell whose order corresponds to a value obtained by dividingthe total cell number N by three. For example, when the total cellnumber is 60, the (N/3)^(th) cell is a 20^(th) cell. Incidentally, whenthe (N/3) is not an integer, the (N/3)^(th) cell is specified byrounding off to the nearest integer. Also, the surface-side cell may be,for example, a cell that belongs to a cell region including the 1^(st)cell to the 20^(th) cell, and may be a cell that belongs to a cellregion including the 1^(st) cell to a 10^(th) cell.

Also, the surface-side cell may be, for example, a cell that belongs toa cell region including a 5^(th) cell to the (N/3)^(th) cell, and may bea cell that belongs to a cell region including the 10^(th) cell to the(N/3)^(th) cell. As mentioned in the later described Reference Examples1 and 2, due to the influence of an exterior package such as a laminatefilm, the short circuit resistance of the 1^(st) cell during a nailpenetration may be high in some cases. Therefore, the surface-side cellmay be specified, excluding the 1^(st) cell and the neighborhood cells.

Meanwhile, the center-side cell is a cell that is located on the centerside rather than the surface-side cell. “Center side” refers to thecentral side in the thickness direction of the stacked cells. Thecenter-side cell is preferably a cell that belongs to a cell regionincluding a ((N/3)+1)^(th) cell to a (2N/3)^(th) cell. Here, the((N/3)+1)^(th) cell refers to a cell next to the (N/3)^(th) cell whennumbered from the 1^(st) cell. Meanwhile, the (2N/3)^(th) cell is a cellwhose order corresponds to a value obtained by dividing the doubledvalue of the total cell number N by three. For example, when the totalcell number is 60, the (2N/3)^(th) cell is a 40^(th) cell. Incidentally,when the (2N/3) is not an integer, the (2N/3)^(th) cell is specified byrounding off to the nearest integer. Also, the center-side cell may be,for example, a cell that belongs to a cell region including the 21^(st)cell to the 40^(th cell.)

Also, a cell region including the 1^(st) cell to the (N/3)^(th) cell isregarded as a cell region A, and a cell region including the((N/3)+1)^(th) cell to the (2N/3)^(th) cell is regarded as a cell regionB. The average contact resistance R_(CA) in the cell region A ispreferably more than the average contact resistance R_(CB) in the cellregion B. The value of R_(CA)/R_(CB) under pressure of 100 MPa is, forexample, 1.5 or more, and may be 10 or more. Meanwhile, the value ofR_(CA)/R_(CB) under pressure of 100 MPa is, for example, 10000 or less.

Also, a cell region including the 1^(st) cell to the 20^(th) cell isregarded as a cell region C, and a cell region including the 21^(st)cell to the 40^(th) cell is regarded as a cell region D. The averagecontact resistance R_(CC) in the cell region C is preferably more thanthe average contact resistance R_(CD) in the cell region D. The value ofR_(CC)/R_(CD) under pressure of 100 MPa is, for example, 1.5 or more,and may be 10 or more. Meanwhile, the value of R_(CC)/R_(CD) underpressure of 100 MPa is, for example, 10000 or less.

Also, in the present disclosure, the contact resistance between thecathode current collector and the anode current collector, underpressure of 100 MPa, in all of the plurality of cells may be 0.037 Ω·cm²or more, may be 0.047 Ω·cm² or more, and may be 0.103 Ω·cm² or more.

Also, in the stacked battery after a nail penetration test, the shortcircuit resistance of the cell with the lowest short circuit resistanceis regarded as R_(Min), and the short circuit resistance of the cellwith the highest short circuit resistance is regarded as R_(Max). Forexample, when a metal active material (particularly Si or an Si alloy)is used as the anode active material, the value of R_(Max)/R_(Min) ispreferably 100 or less, and more preferably 5.0 or less. Incidentally,the nail penetration test is carried out under conditions mentioned inthe later described Reference Examples 1 and 2.

3. Cell

The cell in the present disclosure includes a cathode current collector,a cathode active material layer, a solid electrolyte layer, an anodeactive material layer, and an anode current collector, in this order.The cell is typically a cell utilizing Li ion conductivity (a Li ioncell). Also, the cell is preferably a cell capable of being charged anddischarged (a secondary battery).

(1) Anode Active Material Layer

The anode active material layer includes at least an anode activematerial, and may include at least one of a solid electrolyte material,a conductive material, and a binder as required.

The anode active material is not particularly limited, and examplesthereof may include a metal active material, a carbon active material,and an oxide active material. Examples of the metal active material mayinclude a simple substance of metal and a metal alloy. Examples of themetal element included in the metal active material may include Si, Sn,In, and Al. The metal alloy is preferably an alloy including the abovedescribed metal element as the main component. Examples of the Si alloymay include a Si—Al base alloy, a Si—Sn base alloy, a Si—In base alloy,a Si—Ag base alloy, a Si—Pb base alloy, a Si—Sb base alloy, a Si—Bi basealloy, a Si—Mg base alloy, a Si—Ca base alloy, a Si—Ge base alloy, and aSi—Pb base alloy. Incidentally, the Si—Al based alloy, for example,refers to an alloy including at least Si and Al, may be an alloyincluding only Si and Al, and may be an alloy further including anadditional metal element. It is much the same for the alloys other thanthe Si—Al based alloy. The metal alloy may be a two-component basedalloy, and may be a multicomponent based alloy including three or morecomponents.

Meanwhile, examples of the carbon active material may include amesocarbon microbead (MCMB), a highly oriented pyrolytic graphite(HOPG), a hard carbon, and a soft carbon. Also, examples of the oxideactive material may include a lithium titanate such as Li₄Ti₅O₁₂.

Examples of the shape of the anode active material may include agranular shape. The average particle size (D₅₀) of the anode activematerial is, for example, within a range of 10 nm to 50 μm, and may bewithin a range of 100 nm to 20 μm. The proportion of the anode activematerial in the anode active material layer is, for example, 50% byweight or more, and may be within a range of 60% by weight to 99% byweight.

The solid electrolyte material is not particularly limited, and examplesthereof may include an inorganic solid electrolyte material such as asulfide solid electrolyte material, and an oxide solid electrolytematerial. Examples of the sulfide solid electrolyte material may includeLi₂S—P₂S₅, Li₂S—P₂S₅—LiI, Li₂S—P₂S₅—LiI—LiBr, Li₂S—P₂S₅—Li₂O,Li₂S—P₂S₅—Li₂O—LiI, Li₂S—SiS₂, Li₂S—SiS₂—LiI, Li₂S—SiS₂—LiBr,Li₂S—SiS₂—LiCl, Li₂S—SiS₂—B₂S₃—LiI, Li₂S—SiS₂—P₂S₅—LiI, Li₂S—B₂S₃,Li₂S—P₂S₅-ZmSn (wherein m and n are respectively a positive number; Z isany one of Ge, Zn, and Ga.), Li₂S—GeS₂, Li₂S—SiS₂—Li₃PO₄,Li₂S—SiS₂-Li_(x)MO_(y) (wherein x and y are respectively a positivenumber; M is any one of P, Si, Ge, B, Al, Ga, and In.) Incidentally, theabove described “Li₂S—P₂S₅” refers to a sulfide solid electrolytematerial using a raw material composition including Li₂S and P₂S₅, andit is much the same for other descriptions.

In particular, the sulfide solid electrolyte material is preferablyprovided with an ion conductor including Li, A (A is at least one kindof P, Si, Ge, Al, and B), and S. Further, the ion conductor preferablyincludes an anion structure (PS₄ ³⁻ structure, SiS₄ ⁴⁻ structure, GeS₄⁴⁻ structure, AlS₃ ³⁻ structure, BS₃ ³⁻ structure) of anortho-composition, as the main component of an anion. The reasontherefor is to obtain a sulfide solid electrolyte material with highchemical stability. The proportion of the anion structure of theortho-composition among the total anion structures in the ion conductoris preferably 70 mol % or more, and more preferably 90 mol % or more.The proportion of the anion structure of the ortho-composition may bedetermined by, for example, a Raman spectroscopy, a NMR, and an XPS.

In addition to the ion conductor, the sulfide solid electrolyte materialmay include a lithium halide. Examples of the lithium halide may includeLiF, LiCl, LiBr, and LiI, and among them, LiCl, LiBr, and LiI arepreferable. The proportion of LiX (X=I, Cl, Br) in the sulfide solidelectrolyte material is, for example, within a range of 5 mol % to 30mol %, and may be within a range of 15 mol % to 25 mol %.

The solid electrolyte material may be a crystalline material, and may bean amorphous material. Also, the solid electrolyte material may be aglass, and may be a crystallized class (a glass ceramic). Examples ofthe shape of the solid electrolyte material may include a granularshape.

Examples of the conductive material may include carbon materials such asacetylene black (AB), Ketjen black (KB), carbon fiber, carbon nanotube(CNT), and carbon nanofiber (CNF). Also, examples of the binder mayinclude rubber based binders such as butylene rubber (BR),styrene-butadiene rubber (SBR); and fluoride based binders such aspolyvinylidene fluoride (PVDF).

The thickness of the anode active material layer is, for example, withina range of 0.1 μm to 300 μm, and may be within a range of 0.1 μm to 100μm.

(2) Cathode Active Material Layer

The cathode active material layer includes at least a cathode activematerial, and may include at least one of a solid electrolyte material,a conductive material, and a binder as required.

The cathode active material is not particularly limited, and examplesthereof may include an oxide active material. Examples of the oxideactive material may include a rock salt bed type active material such asLiCoO₂, LiMnO₂, LiNiO₂, LiVO₂, and LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂; aspinel type active material such as LiMn₂O₄, Li₄Ti₅O₁₂, andLi(Ni_(0.5)Mn_(1.5))O₄; and an olivine type active material such asLiFePO₄, LiMnPO₄, LiNiPO₄, and LiCoPO₄. Also, as the oxide activematerial, for example, a LiMn spinel active material represented byLi_(1+x)Mn_(2-x-y)M_(y)O₄ (M is at least one kind of Al, Mg, Co, Fe, Ni,and Zn, and 0<x+y<2), and a lithium titanate may be used.

Also, on a surface of the cathode active material, a coating layerincluding a Li ion conductive oxide may be formed. The reason thereforeis to suppress the reaction between the cathode active material and thesolid electrolyte material. Examples of the Li ion conductive oxide mayinclude LiNbO₃, Li₄Ti₅O₁₂, and Li₃PO₄. The thickness of the coatinglayer is, for example, within a range of 0.1 nm to 100 nm, and may bewithin a range of 1 nm to 20 nm. The coverage of the coating layer onthe cathode active material surface is, for example, 50% or more, andmay be 80% or more.

The solid electrolyte material, the conductive material, and the binderused for the cathode active material layer are respectively in the samecontents as those described in “(1) Anode active material layer” above;thus, the descriptions herein are omitted. Also, the thickness of thecathode active material layer is, for example, within a range of 0.1 μmto 300 μm, and may be within a range of 0.1 μm to 100 μm.

(3) Solid Electrolyte Layer

The solid electrolyte layer is a layer formed between the cathode activematerial layer and the anode current collector. Also, the solidelectrolyte layer includes at least a solid electrolyte material, andmay further include a binder as required. The solid electrolyte materialand the binder used for the solid electrolyte layer are respectively inthe same contents as those described in “(1) Anode active materiallayer” above; thus, the descriptions herein are omitted.

The content of the solid electrolyte material in the solid electrolytelayer is, for example, within a range of 10% by weight to 100% byweight, and may be within a range of 50% by weight to 100% by weight.Also, the thickness of the solid electrolyte layer is, for example,within a range of 0.1 μm to 300 μm, and may be within a range of 0.1 μmto 100 μm.

(4) Cathode Current Collector and Anode Current Collector

The cathode current collector collects currents of the above describedcathode active material layer, and the anode current collector collectscurrents of the above described anode active material layer. The metalelement included in the cathode current collector is not particularlylimited, and examples thereof may include Al, Fe, Ti, Ni, Zn, Cr, Au,and Pt. The cathode current collector may be a simple substance of themetal element, and may be an alloy including the metal element as themain component. Stainless steel (SUS) is an example of the Fe alloy, andSUS304 is preferable.

Examples of the shape of the cathode current collector may include afoil shape and a mesh shape. The thickness of the cathode currentcollector is, for example, 0.1 μm or more, and may be 1 μm or more. Whenthe cathode current collector is too thin, the current collectingfunction may be degraded. Meanwhile, the thickness of the cathodecurrent collector is, for example, 1 mm or less, and may be 100 μm orless. When the cathode current collector is too thick, the energydensity of a battery may be degraded.

The metal element included in the anode current collector is notparticularly limited, and examples thereof may include Cu, Fe, Ti, Ni,Zn, and Co. The anode current collector may be a simple substance of themetal element, and may be an alloy including the metal element as themain component. Stainless steel (SUS) is an example of the Fe alloy, andSUS304 is preferable.

Examples of the shape of the anode current collector may include a foilshape and a mesh shape. The thickness of the anode current collector is,for example, 0.1 μm or more, and may be 1 μm or more. When the anodecurrent collector is too thin, the current collecting function may bedegraded. Meanwhile, the thickness of the anode current collector is,for example, 1 mm or less, and may be 100 μm or less. When the anodecurrent collector is too thick, the energy density of a battery may bedegraded.

Incidentally, the present disclosure is not limited to the embodiments.The embodiments are exemplification, and any other variations areintended to be included in the technical scope of the present disclosureif they have substantially the same constitution as the technical ideadescribed in the claim of the present disclosure and offer similaroperation and effect thereto.

EXAMPLES

The present disclosure will be described in more details. First, in eachof Reference Examples 1 and 2, it was confirmed that the unevenness ofshort circuit resistance among a plurality of cells in a conventionalstacked battery was large.

Reference Example 1

Production of Cathode

Using a tumbling fluidized bed granulating-coating machine (manufacturedby Powrex Corp.), the cathode active material(Li_(1.15)Ni_(1/3)Co_(1/3)Mn_(1/3)W_(0.005)O₂) was coated with LiNbO₃ inthe atmospheric environment. After that, by burning thereof in theatmospheric environment, a coating layer including LiNbO₃ was formed onthe surface of the cathode active material. Thereby, a cathode activematerial having the coating layer on the surface thereof was obtained.

Next, butyl butyrate, 5% by weight butyl butyrate solution of a PVDFbased binder (manufactured by Kureha Corp.), the obtained cathode activematerial, a sulfide solid electrolyte material (Li₂S—P₂S₅ based glassceramic including LiI and LiBr, average particle size D₅₀=0.8 μm), and aconductive material (a vapor-grown carbon fiber, VGCF, manufactured byShowa Denko K. K.) were added into a propylene (PP) container so as tobe cathode active material:sulfide solid electrolyte material:conductivematerial:binder=85:13:1:1 in the weight ratio. Next, the PP containerwas stirred for 30 seconds by an ultrasonic dispersion apparatus (UH-50,manufactured by SMT Corp.) Next, the PP container was agitated for 3minutes by an agitation mixer (TTM-1, manufactured by Sibata ScientificTechnology LTD.), and further, was stirred for 30 seconds by theultrasonic dispersion apparatus to obtain a coating solution.

Next, an Al foil (manufactured by Nippon Foil Mfg. Co. Ltd., a cathodecurrent collector) was prepared. The obtained coating solution waspasted on the Al foil by a blade method using an applicator. The coatedelectrode was dried naturally, and then, was dried at 100° C. for 30minutes on a hot plate to form a cathode active material layer on onesurface of the cathode current collector. Next, the obtained product wascut according to the size of the battery to obtain a cathode.

Production of Anode

Butyl butyrate, 5% by weight butyl butyrate solution of a PVDF basedbinder (manufactured by Kureha Corp.), an anode active material(silicon, manufactured by Kojundo Chemical Lab. Co., Ltd., averageparticle size D₅₀=5 μm), a sulfide solid electrolyte material (Li₂S—P₂S₅based glass ceramic including LiI and LiBr, average particle sizeD₅₀=0.8 μm), and a conductive material (a vapor-grown carbon fiber,VGCF, manufactured by Showa Denko K. K.) were added into a PP containerso as to be anode active material:sulfide solid electrolytematerial:conductive material:binder=55:42:2:1 in the weight ratio. Next,the PP container was stirred for 30 seconds by an ultrasonic dispersionapparatus (UH-50, manufactured by SMT Corp.). Next, the PP container wasagitated for 30 minutes by an agitation mixer (TTM-1, manufactured bySibata Scientific Technology LTD.), and further, was stirred for 30seconds by the ultrasonic dispersion apparatus to obtain a coatingsolution.

Next, as shown in FIG. 6A, a Cu foil (anode current collector 5) wasprepared. The obtained coating solution was pasted on the Cu foil by ablade method using an applicator. The coated electrode was driednaturally, and then, was dried at 100° C. for 30 minutes on a hot plate.Thereby, as shown in FIG. 6B, anode active material layer 2 was formedon one surface of the Cu foil (anode current collector 5). After that,by the similar treatment, anode active material layer 2 was formed onanother surface of the Cu foil (anode current collector 5) as shown inFIG. 6C. Next, the obtained product was cut according to the size of thebattery to obtain an anode.

Production of Solid Electrolyte Layer

Heptane, 5% by weight heptane solution of a butylene rubber based binder(manufactured by JSR Corp.), and a sulfide solid electrolyte material(Li₂S—P₂S₅ based glass ceramic including LiI and LiBr, average particlesize D₅₀=2.5 μm) were added into a PP container. Next, the PP containerwas stirred for 30 seconds by an ultrasonic dispersion apparatus (UH-50,manufactured by SMT Corp.). Next, the PP container was agitated for 30minutes by an agitation mixer (TTM-1, manufactured by Sibata ScientificTechnology LTD.), and further, was stirred for 30 seconds by theultrasonic dispersion apparatus to obtain a coating solution.

Next, an Al foil (manufactured by Nippon Foil Mfg. Co. Ltd.) wasprepared. The obtained coating solution was pasted on the Al foil by ablade method using an applicator. The coated electrode was driednaturally, and then, was dried at 100° C. for 30 minutes on a hot plate.Next, the obtained product was cut according to the size of the batteryto obtain a transfer member including the Al foil and the solidelectrolyte layer.

Production of Evaluation Battery

Each of the two obtained transfer members was placed on the anode activematerial layers formed on the both sides of the anode current collector,and the product was pressed under the pressure of 4 ton/cm² by a coldisostatic pressing method (CIP method). After that, the Al foils of thetransfer members were peeled off. Thereby, as shown in FIG. 6D, solidelectrolyte layers 3 were formed on anode active material layers 2.Next, each of the two above obtained cathodes was placed on the solidelectrolyte layers formed on the both sides of the anode currentcollectors, and the product was pressed under the pressure of 4 ton/cm²by the cold isostatic pressing method (CIP method). Thereby, as shown inFIG. 6E, cathode active material layers 1 and cathode current collectors4 were formed on solid electrolyte layers 3. As described above, atwo-stacked cell was obtained. Further, 30 of the obtained two-stackedcells were stacked, and the obtained product was sealed with an aluminumlaminate film to obtain an evaluation battery.

Reference Example 2

Production of Anode

Butyl butyrate, 5% by weight butyl butyrate solution of a PVDF basedbinder (manufactured by Kureha Corp.), an anode active material (naturalgraphite, manufactured by Nippon Carbon Co., Ltd., average particle sizeD₅₀=10 μm), and a sulfide solid electrolyte material (Li₂S—P₂S₅ basedglass ceramic including LiI and LiBr, average particle size D₅₀=0.8 μm)were added into a PP container so as to be anode active material:sulfidesolid electrolyte material:binder=59:40:1 in the weight ratio. Next, thePP container was stirred for 30 seconds by an ultrasonic dispersionapparatus (UH-50, manufactured by SMT Corp.) Next, the PP container wasagitated for 30 minutes by an agitation mixer (TTM-1, manufactured bySibata Scientific Technology LTD.), and further, was stirred for 30seconds by the ultrasonic dispersion apparatus to obtain a coatingsolution.

Production of Evaluation Battery

A two-stacked cell was obtained in the same manner as in ReferenceExample 1 except that the obtained coating solution was used. Further,an evaluation battery was obtained in the same manner as in ReferenceExample 1 except that 40 of the obtained two-stacked cells were stacked.

[Evaluation]

A nail penetration test was conducted for each evaluation batteryobtained in Reference Examples 1 and 2 under the following conditions.

Charging status: uncharged

Resistance meter: RM3542 manufactured by Hioki E. E. Corp.

Nail: SK (carbon tool steel) material (ϕ): 8 mm, tip angle: 60°)

Speed of the nail: 25 mm/sec

The short circuit resistance of a cell was obtained from a voltageprofile upon the nail penetration. An example of the voltage profile isshown in FIG. 7. As shown in FIG. 7, the voltage of the cell decreasesby penetrating the nail. Here, the initial voltage is referred to as V₀,and the minimum voltage upon the nail penetration is referred to as V.Also, the internal resistance of the cell was measured in advance, andthe internal resistance is referred to as r. Also, the short circuitresistance of the cell is referred to as R. When presuming that all ofthe current generated due to the voltage drop upon the nail penetrationis the short circuit current, a relationship of V/R=(V₀−V)/r isestablished. The short circuit resistance R of the cell may becalculated from this relationship. By compiling the voltage profile ofeach cell, a variation of the short circuit resistance in the thicknessdirection was confirmed. The results thereof are shown in Table 1 andTable 2. Incidentally, the values of the short circuit resistance inTable 1 and Table 2 are relative values when the short circuitresistance of the 1^(st) cell is 1. Also, the cell close to the nailpenetration surface was numbered as the 1^(st) cell.

TABLE 1 <Si> Short circuit N^(th) cell resistance  1 1 10 0.005 20 2885930 38255 40 671141 50 18121 60 0.026

TABLE 2 <C> Short circuit N^(th) cell resistance  1 1 10 0.159 40 0.96860 3.683 80 0.698

As shown in Table 1 and Table 2, in both Reference Examples 1 and 2, theshort circuit resistance of the 1^(st) cell was more than the shortcircuit resistance of the 10^(th) cell. The reason therefor is presumedthat upon the nail penetration, the insulating part of the laminate filmwas dragged in. Also, in Reference Example 1, the short circuitresistance of the 40^(th) cell was more compared to the short circuitresistance of the 1^(st) cell or the 10^(th) cell, and in ReferenceExample 2, the short circuit resistance of the 60^(th) cell was morecompared to the short circuit resistance of the 1^(st) cell or the10^(th) cell. As described above, the short circuit resistance was lessin the surface-side cell, and was more in the center-side cell.Particularly, in Reference Example 1 in which Si was used as the anodeactive material, the unevenness of the short circuit resistance wasextremely large compared to Reference Example 2 in which C was used asthe anode active material.

Experimental Example 1

An Al foil (15 μm thickness, 1N30 manufactured by UACJ Corp.) wasprepared as a cathode current collector, and a Cu foil (12 μm thickness,electrolytic Cu foil manufactured by Furukawa Electric Co., Ltd.) wasprepared as an anode current collector.

Experimental Example 2

A SUS foil (SUS304, 15 μm thickness, manufactured by Toyo Seihaku Co.,Ltd.) was prepared as a cathode current collector, and a Cu foil (12 μmthickness, electrolytic Cu foil manufactured by Furukawa Electric Co.,Ltd.) was prepared as an anode current collector.

Experimental Example 3

A Ti foil (10 μm thickness, TR207C-H manufactured by Takeuchi KinzokuHakufun Kogyo Co., Ltd.) was prepared as a cathode current collector,and a Cu foil (12 μm thickness, electrolytic Cu foil manufactured byFurukawa Electric Co., Ltd.) was prepared as an anode current collector.

Experimental Example 4

An Al foil (15 μm thickness, 1N30 manufactured by UACJ Corp.) wasprepared as a cathode current collector, and a Fe foil (10 μm thickness,manufactured by Takeuchi Kinzoku Hakufun Kogyo Co., Ltd.) was preparedas an anode current collector.

Experimental Example 5

A Fe foil (10 μm thickness, manufactured by Takeuchi Kinzoku HakufunKogyo Co., Ltd.) was prepared as a cathode current collector, and a Cufoil (12 μm thickness, electrolytic Cu foil manufactured by FurukawaElectric Co., Ltd.) was prepared as an anode current collector.

[Evaluation]

The contact resistance between the cathode current collector and theanode current collector each prepared in Experimental Examples 1 to 5was measured. Specifically, as shown in FIG. 8, anode current collector5 was placed on bakelite plate 21, kapton film 22 having a throughsection was placed on anode current collector 5, and cathode currentcollector 4 was placed on kapton film 22. Further, SK material block 23with ϕ11.28 mm was placed on cathode current collector 4 so as tooverlap the through section of kapton film 22 in plan view. In thissituation, the resistance value, when 100 MPa was applied by autograph24, was measured by a resistance meter (RM3542, manufactured by Hioki E.E. Corp.). The results thereof are shown in Table 3.

TABLE 3 Contact resistance Cathode current Anode current [Ω · cm²]collector collector 100 MPa Experimental Al Cu 0.001 Example 1Experimental SUS Cu 0.103 Example 2 Experimental Ti Cu 0.224 Example 3Experimental Al Fe 0.047 Example 4 Experimental Fe Cu 0.037 Example 5

As shown in Table 3, it was confirmed that the contact resistancediffers according to the material of the current collector. From theseresults, it was suggested that, by making the contact resistance of thesurface-side cell relatively high, and by making the contact resistanceof the center-side cell relatively low, the unevenness of short circuitresistance among a plurality of cells may be suppressed.

Experimental Example 6

A heat treatment was conducted to a SUS foil (SUS304, 15 μm thickness,manufactured by Toyo Seihaku Co., Ltd.) under the condition of 200° C.for 1 hour, in the atmosphere, and an oxide layer (oxide layer of SUS)was formed on a surface thereof. The obtained SUS foil was prepared as acathode current collector, and a Cu foil (12 μm thickness, electrolyticCu foil manufactured by Furukawa Electric Co., Ltd.) was prepared as ananode current collector.

Experimental Example 7

A heat treatment was conducted to a SUS foil (SUS304, 15 μm thickness,manufactured by Toyo Seihaku Co., Ltd.) under the condition of 500° C.for 1 hour, in the atmosphere, and an oxide layer (oxide layer of SUS)was formed on a surface thereof. The obtained SUS foil was prepared as acathode current collector, and a Cu foil (12 μm thickness, electrolyticCu foil manufactured by Furukawa Electric Co., Ltd.) was prepared as ananode current collector.

[Evaluation]

The contact resistance between the cathode current collector and theanode current collector each prepared in Experimental Examples 1, 2, 6and 7 was measured. The contact resistance was measured by the methoddescribed above. Also, the resistance value when 1 MPa was applied wasmeasured similarly. The results thereof are shown in Table 4.

TABLE 4 Cathode current Anode current Contact resistance [Ω · cm²]collector collector 1 MPa 100 MPa 1 MPa/100 MPa Experimental Al Cu 0.0020.001 2 Example 1 Experimental SUS Cu 0.892 0.103 8.7 Example 2Experimental SUS (200° C., 1 h) Cu 7.332 0.192 38 Example 6 ExperimentalSUS (500° C., 1 h) Cu 3.501 0.277 12.6 Example 7

As shown in Table 4, the contact resistance was increased by forming theoxide layer on the surface of the current collector. From these results,it was suggested that, by forming the oxide layer on the currentcollector of the surface-side cell, the short circuit resistance of thesurface-side cell may be increased, and as the result, the unevenness ofshort circuit resistance among a plurality of cells may be suppressed.

Also, as described above, in a typical all-solid-type stack battery,since all of the constituting members are solids, a pressure applied tothe stacked battery during a nail penetration test will be extremelyhigh. For example, a high pressure such as 100 MPa or more is applied tothe part where the nail penetrates. Since the situations in a highpressure condition (such as 100 MPa or more) and low pressure condition(such as 1 MPa) are completely different, it is difficult to predict theshort circuit resistance in a high pressure condition, based on theshort circuit resistance in a low pressure condition. Actually, the rateof resistance change (1 MPa/100 MPa) in Experimental Examples 1 and 2,in which the oxide layer was not provided, was about one digit, whereasthe rate of resistance change (1 MPa/100 MPa) in Experimental Examples 6and 7, in which the oxide layer was provided, was about two digits.Accordingly, it is difficult to predict the short circuit resistance ina high pressure condition, based on the short circuit resistance in alow pressure condition.

Experimental Example 8

A paste was produced by mixing N-methylpyrrolidone (NMP) with aconductive material (furnace black, average primary particle size of 66nm, manufactured by Tokai Carbon Co., Ltd.) and PVDF (KF polymer L#9130,manufactured by Kureha Corp.) so as to be conductive material:PVDF=20:80in the volume ratio. An Al foil (15 μm thickness, 1N30 manufactured byUACJ Corp.) was coated with the obtained paste so as the thickness afterdrying to be 10 μm, dried in a drying furnace, and a coating layer wasformed. The obtained Al foil was prepared as a cathode currentcollector, and a Cu foil (12 μm thickness, electrolytic Cu foilmanufactured by Furukawa Electric Co., Ltd.) was prepared as an anodecurrent collector.

Experimental Example 9

A coating layer was formed in the same manner as in Experimental Example8 except that the volume ratio was changed to conductivematerial:PVDF=9:91. The obtained Al foil was prepared as a cathodecurrent collector, and a Cu foil (12 μm thickness, electrolytic Cu foilmanufactured by Furukawa Electric Co., Ltd.) was prepared as an anodecurrent collector.

Experimental Example 10

A coating layer was formed in the same manner as in Experimental Example8 except that the volume ratio was changed to conductivematerial:PVDF=8:92. The obtained Al foil was prepared as a cathodecurrent collector, and a Cu foil (12 μm thickness, electrolytic Cu foilmanufactured by Furukawa Electric Co., Ltd.) was prepared as an anodecurrent collector.

[Evaluation]

The contact resistance between the cathode current collector and theanode current collector each prepared in Experimental Examples 8 to 10was measured. The contact resistance was measured by the methoddescribed above. The results thereof are shown in Table 5.

TABLE 5 Cathode current collector Anode Contact resistance C ratiocurrent [Ω · cm2] Kind [vol. %] collector 100 MPa Experimental C coatedAl 20 Cu 0.360 Example 8 Experimental C coated Al 9 Cu 2.280 Example 9Experimental C coated Al 8 Cu 5.450 Example 10

As shown in Table 5, it was confirmed that the contact resistancediffers according to the content of the carbon material in the currentcollector. From these results, it was suggested that, by making thecontent of the carbon material in the current collector of thesurface-side cell relatively low, and by making the content of thecarbon material in the current collector of the center-side cellrelatively high, the unevenness of short circuit resistance among aplurality of cells may be suppressed.

Experimental Example 11

A paste was produced by mixing N-methylpyrrolidone (NMP) with aconductive material (furnace black, average primary particle size of 66nm, manufactured by Tokai Carbon Co., Ltd.) and PVDF (KF polymer L#9130,manufactured by Kureha Corp.) so as to be conductive material:PVDF=20:80in the volume ratio. An Al foil (15 μm thickness, 1N30 manufactured byUACJ Corp.) was coated with the obtained paste so as the thickness afterdrying to be 3 μm, dried in a drying furnace, and a coating layer wasformed. The obtained Al foil was prepared as a cathode currentcollector, and a Cu foil (12 μm thickness, electrolytic Cu foilmanufactured by Furukawa Electric Co., Ltd.) was prepared as an anodecurrent collector.

Experimental Example 12

A coating layer was formed in the same manner as in Experimental Example11 except that the thickness of the coating layer was changed to 6 μm.The obtained Al foil was prepared as a cathode current collector, and aCu foil (12 μm thickness, electrolytic Cu foil manufactured by FurukawaElectric Co., Ltd.) was prepared as an anode current collector.

Experimental Example 13

A coating layer was formed in the same manner as in Experimental Example11 except that the thickness of the coating layer was changed to 9 μm.The obtained Al foil was prepared as a cathode current collector, and aCu foil (12 μm thickness, electrolytic Cu foil manufactured by FurukawaElectric Co., Ltd.) was prepared as an anode current collector.

Experimental Example 14

A coating layer was formed in the same manner as in Experimental Example11 except that the thickness of the coating layer was changed to 12 μm.The obtained Al foil was prepared as a cathode current collector, and aCu foil (12 μm thickness, electrolytic Cu foil manufactured by FurukawaElectric Co., Ltd.) was prepared as an anode current collector.

[Evaluation]

The contact resistance between the cathode current collector and theanode current collector each prepared in Experimental Examples 11 to 14was measured. The contact resistance was measured by the methoddescribed above. The results thereof are shown in Table 6.

TABLE 6 Contact Cathode current collector Anode resistance Coatingthickness current [Ω · cm2] Kind [μm] collector 15 MPa Experimental Ccoated Al 3 Cu 0.4 Example 11 Experimental C coated Al 6 Cu 0.6 Example12 Experimental C coated Al 9 Cu 1.2 Example 13 Experimental C coated Al12 Cu 1.4 Example 14

As shown in Table 6, it was confirmed that the contact resistancediffers according to the thickness of the coating layer. From theseresults, it was suggested that, by making the thickness of the coatinglayer in the current collector of the surface-side cell relatively high,and by making the thickness of the coating layer in the currentcollector of the center-side cell relatively low, the unevenness ofshort circuit resistance among a plurality of cells may be suppressed.

Experimental Example 15

A paste was produced by mixing N-methylpyrrolidone (NMP) with aconductive material (furnace black, average primary particle size of 66nm, manufactured by Tokai Carbon Co., Ltd.), a filler (alumina, CB-P02manufactured by Showa Denko K. K.) and PVDF (KF polymer L#9130,manufactured by Kureha Corp.) so as to be conductivematerial:filler:PVDF=10:60:30 in the volume ratio. The both sides of anAl foil (15 μm thickness, 1N30 manufactured by UACJ Corp.) were coatedwith the obtained paste so as each thickness after drying to be 10 μm,dried in a drying furnace, and coating layers were formed. Accordingly,an electrode was prepared.

Experimental Example 16

An electrode was prepared in the same manner as in Experimental Example15 except that the volume ratio was changed to conductivematerial:filler:PVDF=8:70:22.

Experimental Example 17

An electrode was prepared in the same manner as in Experimental Example15 except that the volume ratio was changed to conductivematerial:filler:PVDF=8:62:30.

[Evaluation]

A current breaking performance test was conducted for the electrode eachprepared in Experimental Examples 15 to 17. Specifically, as shown inFIG. 9, test piece 30 including current collector (Al foil) 31 andcoating layers 32 formed on the both sides thereof was prepared, Al foil41 and SK material block 42 were placed in sequence on each of the twocoating layers 32, and was bound by 60 MPa. A direct current wassupplied in this situation, the resistance was determined from thechange in voltage at that time. The measurement was conducted in anelectric furnace, and the temperatures in the electric furnace were 25°C. and 200° C. The results thereof are shown in Table 7. Incidentally,the resistance values in Table 7 are relative values when the resistancein Experimental Example 15 at 25° C. is 1.

TABLE 7 Current collector Resistance Kind Coating composition (vol.ratio) 25° C. 200° C. R₂₀₀/R₂₅ Experimental C coated Al C:Al₂O₃:PVDF =10:60:30 1 11.6 11.6 Example 15 Experimental C coated Al C:Al₂O₃:PVDF =8:70:22 8.3 116 1 4.0 Example 16 Experimental C coated Al C:Al₂O₃:PVDF =8:62:30 5.1 79.4 15.6 Example 17

As shown in Table 7, in Experimental Examples 15 to 17, every R₂₀₀/R₂₅was 10 times or more, exhibiting high current breaking performance. Whenthe temperature within a battery is raised due to a short circuit, forexample, a current will be shut off by the resistance also beingincreased. Also, it was confirmed that the resistance differs accordingto the composition of the coating layer. From these results, it wassuggested that, by making the current breaking performance of thecurrent collector of the surface-side cell relatively high, and bymaking the current breaking performance of the current collector of thecenter-side cell relatively low, the unevenness of short circuitresistance among a plurality of cells may be suppressed.

Experimental Example 18

An Al foil (15 μm thickness, 1N30 manufactured by UACJ Corp.) wasprepared as a cathode current collector. Meanwhile, a paste was producedby mixing N-methylpyrrolidone (NMP) with a conductive material (furnaceblack, average primary particle size of 66 nm, manufactured by TokaiCarbon Co., Ltd.), a filler (alumina, CB-P02 manufactured by Showa DenkoK. K.) and PVDF (KF polymer L#9130, manufactured by Kureha Corp.) so asto be conductive material:filler:PVDF=10:60:30 in the volume ratio. A Cufoil (12 μm thickness, electrolytic Cu foil manufactured by FurukawaElectric Co., Ltd.) was coated with the obtained paste so as thethickness after drying to be 10 μm, dried in a drying furnace, and acoating layer was formed. The obtained Cu foil was prepared as an anodecurrent collector.

Experimental Example 19

A paste was produced by mixing N-methylpyrrolidone (NMP) with aconductive material (furnace black, average primary particle size of 66nm, manufactured by Tokai Carbon Co., Ltd.), a filler (alumina, CB-P02manufactured by Showa Denko K. K.) and PVDF (KF polymer L#9130,manufactured by Kureha Corp.) so as to be conductivematerial:filler:PVDF=10:60:30 in the volume ratio. An Al foil (15 μmthickness, 1N30 manufactured by UACJ Corp.) was coated with the obtainedpaste so as the thickness after drying to be 10 μm, dried in a dryingfurnace, and a coating layer was formed. The obtained Al foil wasprepared as a cathode current collector. Meanwhile, a Cu foil (12 μmthickness, electrolytic Cu foil manufactured by Furukawa Electric Co.,Ltd.) was prepared as an anode current collector.

[Evaluation]

The contact resistance between the cathode current collector and theanode current collector each prepared in Experimental Examples 18 and 19was measured. The contact resistance was measured by the methoddescribed above. The results thereof are shown in Table 8.

TABLE 8 Contact resistance Cathode current Anode current [Ω · cm²]collector collector 100 MPa Experimental Al C coated Cu 0.156 Example 18Experimental C coated Al Cu 0.019 Example 19

As shown in Table 8, it was confirmed that the contact resistancediffers by changing the current collector on which the coating layer isformed. From these results, it was suggested that, by making the contactresistance of the surface-side cell relatively high, and by making thecontact resistance of the center-side cell relatively low, theunevenness of short circuit resistance among a plurality of cells may besuppressed.

REFERENCE SIGNS LIST

-   1 cathode active material layer-   2 anode active material layer-   3 solid electrolyte layer-   4 cathode current collector-   5 anode current collector-   10 cell-   100 stacked battery-   110 nail

What is claimed is:
 1. A stacked battery comprising: a plurality ofcells in a thickness direction, wherein the plurality of cells areelectrically connected in parallel; each of the plurality of cellsincludes a cathode current collector, a cathode active material layer, asolid electrolyte layer, an anode active material layer, and an anodecurrent collector, in this order; the stacked battery includes asurface-side cell that is located on a surface side of the stackedbattery, and a center-side cell that is located on a center side ratherthan the surface-side cell; and a contact resistance between the cathodecurrent collector and the anode current collector in the surface-sidecell is more than a contact resistance between the cathode currentcollector and the anode current collector in the center-side cell. 2.The stacked battery according to claim 1, wherein at least one of thecathode current collector and the anode current collector in thesurface-side cell includes, on a surface thereof, an oxide layer.
 3. Thestacked battery according to claim 1, wherein the cathode currentcollector in the surface-side cell includes, on a surface thereof, afirst coating layer containing a carbon material, and the cathodecurrent collector in the center-side cell includes, on a surfacethereof, a second coating layer containing a carbon material.
 4. Thestacked battery according to claim 3, wherein a content of the carbonmaterial in the first coating layer is less than a content of the carbonmaterial in the second coating layer.
 5. The stacked battery accordingto claim 3, wherein a thickness of the first coating layer is more thana thickness of the second coating layer.
 6. The stacked batteryaccording to claim 3, wherein, when a proportion of a resistance R₂₀₀ at200° C. with respect to a resistance R₂₅ at 25° C. is R₂₀₀/R₂₅, theR₂₀₀/R₂₅ value of the first coating layer is more than the R₂₀₀/R₂₅value of the second coating layer.
 7. The stacked battery according toclaim 1, wherein the anode current collector in the surface-side cellincludes, on a surface thereof, a third coating layer containing acarbon material, and the anode current collector in the center-side cellincludes, on a surface thereof, a fourth coating layer containing acarbon material.
 8. The stacked battery according to claim 7, wherein acontent of the carbon material in the third coating layer is less than acontent of the carbon material in the fourth coating layer.
 9. Thestacked battery according to claim 7, wherein a thickness of the thirdcoating layer is more than a thickness of the fourth coating layer. 10.The stacked battery according to claim 7, wherein, when a proportion ofa resistance R₂₀₀ at 200° C. with respect to a resistance R₂₅ at 25° C.is R₂₀₀/R₂₅, the R₂₀₀/R₂₅ value of the third coating layer is more thanthe R₂₀₀/R₂₅ value of the fourth coating layer.
 11. The stacked batteryaccording to claim 1, wherein: one of the cathode current collector andthe anode current collector in the surface-side cell includes, on asurface thereof, a coating layer containing a carbon material, andanother does not include, on a surface thereof, a coating layercontaining a carbon material; and one of the cathode current collectorand the anode current collector in the center-side cell includes, on asurface thereof, a coating layer containing a carbon material, andanother does not include, on a surface thereof, a coating layercontaining a carbon material in reverse to the surface-side cell. 12.The stacked battery according to claim 1, wherein, when each of theplurality of cells is numbered as 1^(st) cell to N^(th) cell, in whichN≥3, in order along the thickness direction of the stacked battery, thesurface-side cell is a cell that belongs to a cell region A including1^(st) cell to (N/3)^(th) cell.
 13. The stacked battery according toclaim 12, wherein the center-side cell is a cell that belongs to a cellregion B including ((N/3)+1)^(th) cell to (2N/3)^(th) cell.
 14. Thestacked battery according to claim 13, wherein an average value of thecontact resistance in the cell region A is more than an average value ofthe contact resistance in the cell region B.
 15. The stacked batteryaccording to claim 1, wherein, when each of the plurality of cells isnumbered as 1^(st) cell to N^(th) cell, in which N≥60, in order alongthe thickness direction of the stacked battery, the surface-side cell isa cell that belongs to a cell region C including 1^(st) cell to 20^(th)cell.
 16. The stacked battery according to claim 15, wherein thecenter-side cell is a cell that belongs to a cell region D including21^(st) cell to 40^(th) cell.
 17. The stacked battery according to claim16, wherein an average value of the contact resistance in the cellregion C is more than an average value of the contact resistance in thecell region D.
 18. The stacked battery according to claim 1, wherein theanode active material layer includes Si or a Si alloy as an anode activematerial.